1. Field of the Invention
The present invention relates to an optical interference apparatus and position detection apparatus which detect variations in the position of an object in a noncontact state.
2. Related Background Art
FIG. 1 is a view showing the arrangement of an interferometer using a conventional semiconductor laser light source. An optical head 1 includes a semiconductor laser light source 2, a condenser lens 3, an unpolarizing beam splitter 4, and a probe-type polarizing member 5 made of glass and protruding from the optical head 1. The condenser lens 3, unpolarizing beam splitter 4, and probe-type polarizing member 5 are sequentially arranged on the optical path of the optical head 1. The probe-type polarizing member 5 has an inclined split surface 5a and a reflecting surface 5b formed in the transmitting direction of the split surface 5a. An object T to be detected is positioned in the reflecting direction of the split surface 5a. A polarizing plate 6 and sensor 7 are arranged in the reflecting direction of the unpolarizing beam splitter 4 in the optical head 1.
A laser beam L1 emitted from the semiconductor laser light source 2 is transmitted through the condenser lens 3 and unpolarizing beam splitter 4 and incident on the probe-type polarizing member 5. This light beam is split into P and S waves at the split surface 5a of the probe-type polarizing member 5. A light beam L2 transmitted through the split surface 5a is reflected as a reference beam by the reflecting surface 5b and returns to the initial optical path. Meanwhile, a light beam L3 reflected by the split surface 5a converges as a measurement beam onto the object T and is scattered/reflected by the rough surface of the object T. This light returns as scattered light to the initial optical path. The reference beam L2 and measurement beam L3 are reflected by the unpolarizing beam splitter 4 toward the sensor 7.
Each of the reference beam L2 and measurement beam L3 is linearly polarized light having planes of polarization that are perpendicular to each other. When the object T relatively moves in the optical axis direction, each light beam becomes rotating circularly polarized light. When these rotating circularly polarized light beams are incident on the polarizing plate 6, an optical interference signal with variations in intensity can be obtained. In this case, since the converging point of the measurement beam L3 on the object T is equal in wave-optics optical path length to the reflecting surface 6b by which the reference beam L2 is reflected, a maximum interference signal can be obtained on the sensor 7.
Letting xcex be the wavelength of a laser beam from the semiconductor laser light source 2, a sine wave output having one period with xcex/2 is obtained as the object T moves. More specifically, if xcex=780 nm, the sensor 7 outputs a sine wave signal with 780/2=390 nm. When this signal is electrically divided by 1,000, a high resolution of 0.39 nm can be obtained.
Since the probe-type polarizing member 5 has a narrow structure with a small diameter, the probe-type polarizing member 5 need only protrude from the optical head 1. Even if, therefore, the object T is located in a mechanically complicated portion, measurement can be easily performed by inserting only the probe-type polarizing member 5. In addition, since the reference beam L2 is reflected by the reflecting surface 5b of the probe-type polarizing member 5, this optical path is not exposed to the air. This structure is therefore robust against environmental changes. Furthermore, since the optical system is configured to focus a light beam into a small spot at a position where a maximum coherence is obtained, even if the object T has a considerably rough surface, the surface can be handled as a mirror surface. That is, no restrictions are imposed on the objects to be measured.
In the prior art described above, after the light beam L1 is split by the split surface 5a, the reference beam L2 is transmitted through the glass and reflected by the reflecting surface 5b, and the measurement beam L3 is transmitted through the glass and air and reflected by the surface of the object T. For this reason, the geometrical-optics optical path length differs from the wave-optics optical path length. That is, the reference beam L2 and measurement beam L3 differ in the distances to the central positions of spherical waves. As a consequence, a concentric interference pattern P like the one shown in FIG. 2 is generated at the position of the sensor 7.
To obtain a high-contrast electrical signal from the sensor 7, an aperture 8 aiming at the center of the concentric interference pattern P in FIG. 2 is required. As the measurement beam L3, a convergent light beam is used to facilitate measuring the rough surface of the object T. However, the amount of light reflected by the rough surface toward the sensor 7 is small. In addition, since the aperture 8 aims at the center of the concentric interference pattern P, a light loss occurs, resulting in a decrease in S/N ratio. As a consequence, a sufficient precision cannot be ensured upon electrically dividing a signal to increase the resolution.
The present invention has been made to solve the above problem, and has as its object to provide an optical interference apparatus and position detection apparatus which can obtain a signal with a high S/N ratio and improve measurement precision.
The above and other objects, features, and advantages of the present invention will be apparent from the following detailed description in conjunction with the accompanying drawings and the appended claims.